1. Field of the Invention
The present invention relates to an optical amplifier used for an optical transmission system, which transmits a wavelength division multiplexing (WDM) light, particularly to the optical amplifier having a wide input dynamic range.
2. Related Art
In the optical amplifier (hereinafter referred to as WDM optical amplifier) used for the WDM optical transmission system in which signal lights having the different wavelengths are multiplexed to perform the communication, rare-earth doped fiber optical amplifier becomes mainly commercially practical because of low noise figure (NF), high output, and high reliability.
As shown in a system configuration of FIG. 13, a signal light input level per one wavelength of the WDM optical amplifier fluctuates depending on a length of transmission line provided on an input side and a loss difference per unit length of the transmission line, or a temperature characteristic or aged degradation of insertion loss of various optical components arrange on the transmission line.
Therefore, a function that even if an inputted signal light level fluctuates a predetermined output level and output wavelength flatness are kept is very important for the WDM optical amplifier, and a technology for realizing the function is necessary to the construction of the WDM optical transmission system. This is because the difference in signal light power becomes factors (for example, non-linear characteristics of the WDM light and S/N degradation) limiting a transmission distance of the WDM light. It is also important that the function is realized by the single WDM optical amplifier having wider input dynamic range. When the single WDM optical amplifier having wider input dynamic range is realized, the number of total menus (kinds) of the optical amplifier can be reduced in the WDM optical transmission system, which results in simplification of menu management/operation of the optical amplifier and the reduction in inventory number of the optical amplifier. Therefore, the WDM optical transmission system can efficiently be constructed.
Conventionally, in order to realize the characteristics required for the WDM optical amplifier, there is known the technology, in which a variable optical attenuator (VOA) which seemingly contradicts amplification is arranged between a fore-stage amplification unit and a post-stage amplification unit and output level deviation is reduced among signal lights of the wavelengths included in the WDM light while the signal light output level is kept constant with respect to the fluctuation in signal light input level, and this technology becomes a standard configuration of the optical amplifier in the WDM optical transmission system (for example, see Japanese Patent No. 3551418).
There is also reported a technology, in which a variable gain equalizer (VGEQ) is arranged between the fore-stage amplification unit and the post-stage amplification unit and a change in output wavelength characteristic generated by the fluctuation in signal light input level is compensated by adding the loss to each wavelength of the WDM light (for example, see Laurence Lolivier et al., “DGE-Based Variable Gain EDFA Improves Both Gain Flatness and Noise Figure for a 70° C. Temperature Operating Range”, OFC/NFOEC 2005 Conference, OThL4).
However, when the WDM optical amplifier (hereinafter, abbreviated to as inter-stage VOA) in which the variable optical attenuator is arranged between the stages of the amplification units and the WDM optical amplifier (hereinafter, abbreviated to as inter-stage VGEQ) in which the variable gain equalizer is arranged between the stages of the amplification units are compared to each other, both the inter-stage VOA and the inter-stage VGEQ have advantages and disadvantages with respect to a control method, the noise figure (NF), and device cost.
Specifically, for the control method, the control method applied to the inter-stage VOA is as follows: A gain is kept constant by monitoring total power of the inputted and outputted WDM light, and an optical attenuation amount of the variable optical attenuator provided between the stages is changed according to the fluctuation in signal light input level, which holds the predetermines signal light output level which the flatness of the output wavelength characteristic is maintained. On the other hand, the control method in which the insertion loss of the variable gain equalizer for the signal light having each wavelength is changed such that the output wavelength characteristic becomes flat is applied to the inter-stage VGEQ. By comparison, while the control method of the inter-stage VOA is relatively simple because at least the gain (output-input) is controlled by monitoring the input and output in the total power of the WDM light, the control method of the inter-stage VGEQ becomes complicated because it is necessary that the insertion loss is separately controlled in each wavelength range according to the signal light having each wavelength.
For NF, as shown in FIG. 14, because the control in which the attenuation amount of the variable optical attenuator is increased as the signal light input level is increased in the inter-stage VOA, NF is steeply degraded in a region where the signal light input level is high. On the other hand, in the inter-stage VGEQ, because the change in insertion loss of the variable gain equalizer becomes small for the increase in signal light input level when compared with the inter-stage VOA, the NF degradation is small in the region where the signal light input level is high. When a range where the NF degradation is lower than 1 dB for the increase in signal light input level by 1 dB is considered, as shown in FIG. 14, a position at which an NF curved line represented by a solid line and an NF degradation 1 dB line come into contact with each other becomes a maximum value of the signal light input level, so that the inter-stage VGEQ can realize the wider input dynamic range when compared with the inter-stage VOA.
In the inter-stage VGEQ, because the insertion loss value of the variable gain equalizer is large (for example, 8 dB), NF is largely degraded in the region where the signal light input level is low when compared with the inter-stage VOA.
For the device cost, while the variable optical attenuator is inexpensive, the variable gain equalizer is expensive (for example, about 30 times).
In consideration of the above features of the related art, it is desirable to establish the new technology in which the ease of control and low cost which are the advantages of the inter-stage VOA and the wider effective input dynamic range which is the advantage of the inter-stage VGEQ are simultaneously realized.